A gas insulated switchgear (GIS) is a device used to disconnect and/or reconnect high-voltage power lines in power transforming stations, power generation plants, power receiving equipments and the like. Such gas insulated switchgear includes such as a circuit breaker, a circuit disconnector, bus lines, a lightning protector, an instrumental transformer and/or a grounding device housed in a single grounded container filled with a high-insulation gas.
Compared with a common air insulated switchgear, a gas insulated switchgear can be made significantly smaller. In recent years, due to the escalating of land prices, it is difficult to assure a greater installation space, and thereby, such a gas insulated switchgear which can be made smaller has been employed and installed in many electrical switchgear stations.
The gas circuit breaker is a device that can interrupt an electric line when the line is subjected to a short circuit, an over current or an earth fault. The gas circuit breaker is configured to blow gas to an electric arc discharged between electrode contacts of a circuit breaker when interrupting the current so as to extinct (extinguish) the arc discharge. Up to now, various efforts have been made in the gas circuit breaker to blow arc-extinguishing gas to an electric arc discharged between a movable contactor and a fixed contactor at the time of circuit interruption so as to efficiently extinguish the electric arc in performing the circuit interruption operation (see PTD 1 (Japanese Patent Laying-Open No. 7-312155) and PTD 2 (Japanese Patent Laying-Open No. 2001-155595)).
The insulation gas to be used in the gas insulated switchgear (GIS) and the gas circuit breaker is generally sulfur hexafluoride (SF6). Sulfur hexafluoride has a high insulation property, and the insulation strength thereof is up to three times as that of air. In addition, sulfur hexafluoride is inert and high in thermal conductivity, which makes it possible to rapidly cool down the electrodes overheated by the discharged electric arc.
Currently, a main interrupting approach in the gas circuit breaker is puffer-type circuit interruption in which a piston is driven to operate in conjunction with the disconnection operation of electrodes so as to blow insulation gas such as sulfur hexafluoride to the electrodes. Generally, in order to efficiently interrupt the circuit with an operating force under a small current, a heating chamber (thermal puffer) is provided for the purpose of utilizing the heat of the electric arc to increase the pressure of the insulation gas or a pair of cylinder and piston (machine puffer) is provided for the purpose of decreasing the volume of the insulation gas through mechanical forces so as to increase the pressure of the insulation gas. In recent years, due to the demand for increasing the interruption capacity in response to the increase of interruption current or the decrease in interruption points or the request for making the device smaller, the processing energy per unit volume in an arc-extinguishing chamber of a gas circuit breaker increases, and thereby, the arc-extinguishing performance is required to be made more powerful than the conventional one.
In the puffer-type gas circuit breaker mentioned above, there has been proposed as one of the promising solutions for achieving a powerful arc-extinguishing performance by disposing inside the arc-extinguishing chamber a resin material which decomposes at the exposure to an electric arc and generates gas to extinguish the electric arc.
In such a gas circuit breaker, an arc-extinguishing insulation material molded product contributing to arc-extinguishing is disposed in the periphery of contacts of electrodes to be disconnected or connected. When the arc-extinguishing insulation material molded product is exposed to the light and heat from an electric arc, the constituent material of the molded product itself decomposes to generate gas, and the generated gas promotes the extinguishing of the electric arc through blowing so as to cool down the electric arc, through increasing the pressure of insulation gas so as to weaken the electric arc, or through increasing insulation resistance.
For example, in the conventional gas circuit breaker shown in PTD 3 (Japanese Patent Laying-Open No. 2003-297200), a member made of a polymer is disposed inside a heating chamber, and when the member is heated by an electric arc in the arc space, it generates evaporation gas containing no oxygen in the chemical composition thereof to enhance the pressure increase in the heating chamber.
PTD 4 (Japanese Patent Laying-Open No. 11-329191) illustrates resins composed of carbon atoms and hydrogen atoms such as polystyrene, polyethylene, polypropylene and polymethyl pentene as examples of materials which generate evaporation gas when being heated by an electric arc. Since each of these resins can generate hydrogen gas or hydrocarbon gas having excellent thermal diffusion effect, it is possible for it to efficiently cool down the electric arc and thereby exhibit excellent arc-extinguishing performance.
PTD 4 further illustrates resins such as polyoxymethylene, polyethylene, polypropylene, polytetrafluoroethylene and melamine as examples of materials for generating evaporation gas.
However, in the above-mentioned prior arts, when the decomposition gas generated at the exposure to an electric arc is mixed with a blowing gas to perform arc extinguishing, after the circuit interruption is finished and the blowing gas is cooled to approximately several hundred degrees, water moisture may be produced therefrom. The main reason thereof is because the water moisture is formed from oxygen and hydrogen atoms in the polymer. It is known that in the gas circuit breaker, when the content of water moisture increases from several hundred PPM to several thousand PPM, the insulation member made of insulation materials will be deteriorated. Therefore, in a conventional gas circuit breaker, the insulation performance and the blocking performance may not be achieved sufficiently after the current interruption. Furthermore, the water moisture dissolves fluoride compound generated as a decomposition product in the current interruption and turn into hydrofluoric acid, which may corrode the metal components.
Thus, in the conventional gas circuit breaker, polytetrafluoroethylene resin that does not cause the insulation deterioration of the insulation member is often used as a material for generating the decomposition gas.
However, in order to meet the demand for increasing the interruption capacity in response to the increase of interruption current or the decrease in interruption points or to meet the demand for making the device smaller, the gas generated from polytetrafluoroethylene resin is not sufficient in pressure, an arc-extinguishing insulation material which has a higher arc-extinguishing performance is required.